The best place to get aquarium stuff (and an amazing variety of fish) is in Dongdaemun's pet district. I don't remember the exit, but just follow the road along the Chonge-chon (sp?) stream path and you'll see it.

There's one shop there where the owner Sang-Mi speaks pretty good English. He'll set you up with everything you need and is able to deliver anywhere in Seoul.

Even better (given the fact that many are here just for a year or two) he'll take back the stuff (little or no money back, I think, but free pickup, anyway), including the fish, when you're ready to leave.

I bought a fully-kitted 42 gallon tank from him (including stand. gravel, filter, heater, fish, etc) for about W400,000. Given that I plan to stay here for another year, that works out to a rental of about W35,000/month. Not too shabby.

He delivered it the same day, set it up for me (took him about an hour), added some chemicals, and it was ready for fish (South American cichlids) after about three hours of circulation. No problems at all with the fishies two months later -- very healthy!

Go to Dongademun station, find the pet district (easy to find) and give him a call at 011-9892-6840 for specific directions to his shop. You can also check out his site http://www.ggogiya.co.kr/, although its all in Korean.